Baseline susceptibility to imidacloprid and cross resistance patterns in Colorado potato beetle (Coleoptera : Chrysomelidae) populations

During 1995-1998, we tested 134 geographically discrete populations of Colo rado potato beetle, Leptinotarsa decemlineata (Say), from the United States , Canada, Germany. France, and Poland for susceptibility to imidacloprid. N eonates were assayed on potato-based agar diet incorporated with imidaclopr id and exposed on filter paper to esfenvalerate, azinphosmethyl, and carbof uran to characterize cross-resistance. In all 4 yr, Long Island populations were the most tolerant to imidacloprid, with LC(50)s ranging up to 29 time s higher than the most susceptible populations. Responses to imidacloprid d id not change significantly on farms where populations were assayed over ti me, except for those from Long Island, which doubled in overall tolerance t o imidacloprid since 1995. Much of this tolerance was already present befor e imidacloprid was used on Long Island. Correlative analysis of the populat ions tested over the 4 yr indicated positive cross-resistance patterns with esfenvalerate and azinphosmethyl. This response was probably caused by pre existing metabolic and excretion mechanisms selected by previous exposure. There was no significant pattern of cross-resistance with carbofuran or ben sultap. Regression slopes were also significantly negatively correlated wit h LC50 values for imidacloprid, indicating higher heterogeneity, which coul d lead in further resistance development. We discuss the relative sensitivi ty of diet-incorporated assays with neonates compared with other bioassay s tudies. Based on a pooled group of susceptible populations tested in 1995, a baseline LC50 of 0.39 ppm and a discriminating concentration of 8 ppm wer e suggested to detect early stages of resistance in "suspect" populations. We also suggest application strategies for imidacloprid that reduce selecti on pressure.